Overall had fun playing. Nice sound effects and I also liked the boss design. Reached level 9. The initial levels were easy (that is good to get to know how game works), diffuculty progression on later levels was also fine. I got stuck for a while on level 8 and got frustrated a little by having to repeat all of the level every time to get to the place I needed to practise. You could add some life refreshes somewhere on levels maybe, so player can take more than 3 hits on one try, or checkpoint in the middle of the level. But maybe I'm just bad at platforming :)
The whole game brings up Little Big Planet vibes. Maybe adding more interactable things like buttons, triggers or movable objects to enable logic riddles would make the game mechanics more interesting. Also a well animated character is important for platformer games. Walk animation make crab look like it is floating and breaking jump animation where needed would also make difference (When you jump and hit ceiling and fall to ground faster, the jump animation still playes while crab is already on ground).
I also found that the rotating platforms can get you at really high speed when jumping on them at the right moment. Maybe that is a good developement direction for some harder levels?
Good luck on further developement :)